Depositing epitaxial layer of silicon from a vapor mixture of sih4 and h3



April 6, 1965 A. MAYER ETAL DEPOSITING EPITAXIAL LAYER OF SILICON FROM A VAPOR MIXTURE OF 81H4 AND H2 Filed Sept. 9, 1963 3 Sheets-Sheet l H547 Maffe ane; @0A/ 70 naar /zmc F02 Maar/awww /A/ FwW//Va HmeaE/v, M/e/vr KEIN/d5 h/FEQ TEMPEETMZE 7'0 46007 /000 #50 d INVENTORS APlil 6, 1955 A. MAYER ETAL 3,177,100

DEPOSITING EPITAXIAL LA IION f WMM 37 l' mi y LELE April 6, 1965 DEPOSIT FROM A Filed Sept. 9, 1963 l-g. 4a.

. MAYER ETA PITAXIAL LAYER SILICON R MIXTURE OF SLH4 AND H2 Sheets-Sheet 3 IN VEN TORS lreducing the halogen compound with hydrogen.

United States Patent() 3,177,100 DEPOSITING EPITAXIAL LAYER F SILCN FRM A VAPR MHXTURE F SiH., AND H2 Alfred Mayer, Plainfield, and Joseph Leslry, Cranford, NJ., assignors to Radio Corporation of America, a corporation of Delaware Y Filed Sept. 9, 1963, Ser. No. 307,570

6 Claims. (Cl. 14S-175) This invention relates to improved methods of manufacturing improved semiconductor devices, and more particularly, to improved methods of fabricating epitaxial silicon layers on semiconductive wafers.

vEpitaxial layers on semiconductive wafers have been fabricated by forming a halogen compound of la. semiconductor such as germanium, silicon, and the like, then passing the vapors of the particular semiconductor, and The vaporized halides tend to transport conductivity modiliers, so that the junctions between the substrate Wafer and the epitaxial layer are not as abrupt, and the resistivity of the epitaxial layer is not as high or as uniform, as is desirable for some device applications..

Attempts have also been made to deposit epitaxial silicon layers on a semiconductive wafer by heating the wafer in an ambient of silane to a temperature high enough to decompose the silane. These attempts have not hitherto produced satisfactory epitaxial layers, and the method has been regarded as unsafe, because of the explosive nature of silane. The term silane is used throughout to denote silicon tetrahydride, SiH4.

It is advantageous to form epitaxial layers of semiconductive materials with a mirror-smooth surface. It is also desirable that the epitaxial layers formed be substantially monocrystalline. tageous to produce epitaxial layers with a high degree of structural perfection.

Accordingly, it is an object of this invention to provide an improved method of fabricating improved semiconductor devices.

Another object of the invention is to provide an mproved` method of depositing an epitaxial layer on semiconductive wafers.

Still anothervobject is to provide an improved method of depositing epitaxial silicon layers having a mirrorsmooth surface on a semiconductive wafer.

But another object is to provide an improved method of depositing on a semiconductive wafer an epitaxial silicon layer having a high degree of structural perfection.

These and other objects and advantages are attained by a process that includes the step of heating the semiconductive wafer in an ambient consisting essentially of hydrogen and 0.01 to 1.5 volume percent silane. Advantageously the surface of the semi-conductive wafer is cleaned, for example, by means of an etchant which leaves a smooth surface on the wafer, and by heating the Wafer in hydrogen, just before the step of heating the wafer in a hydrogen-silane ambient. According to one embodiment, a gaseous conductivity modier is added to the hydrogen-silane ambient.

It is particularly advan ICSCV FIGURE 4 is a cross-sectional view of a wafer at successive steps in another method of fabricating a semiconductor device.

Eixample I A wafer 30 (FIGURE 3a) of monocrystalline semiconductive material is prepared with two opposing major faces 31 and 33. The exact size, shape and resistivity of wafer 30 is not critical. In this example, wafer 30 is about 30 mils square, 4 mils thick, and consists of N-t-ype silicon having a resistivity of about .001 ohm-cm. Preferable, wafer 30 is lapped and polished, then lightly etched to remove work damage on its surface, so that major wafer faces 31 and 33 are smooth and at.

Apparatus 10 (FIGURE 2) comprises a reaction vessel or deposition chamber 11 having an inlet tube 15, au outlet tube, a water jacket 12, and an RF coil 13 surrounding the Water jacket. Reaction vessel 11 includes a susceptor block 14 of high conductivity material, which is used to provide additional coupling with the RF coil 14 because the mass of the substrate'wafer 30 is small. In this example, susceptor block 14 consists of high conductivity silicon. Reaction vessel 11 and its associated tubing are preferably made of a refractory insulating material such as fused quartz.

Feeding into inlet tube 15 are four gas sources; ay

cylinder 17 consisting of a mixture of silane and hydrogen; a source 18 of pure dry high purity hydrogen, such as that obtained by diffusion through palladium; a cylinder 19 containing an arsine-hydrogen mixture; and a cylinder 20 containing a borane-hydrogen mixture. The flow of gases in the apparatus 10 is controlled by a plurality of valves 23. vAdvantageously, flow meters 24 may be utilized to monitor the rates of flow of each gas.

Outlet tube 16 is connected to a T-shaped tube 26, one end of which leads to an exhaust while the other leads to a vacuum pump 22 and a vacuum gauge 21.

The silicon wafer 30vis positioned on susceptor block Y 14 in apparatus 10; Reaction vessel 11 is evacuated by While continuing the flow of hydrogen, high frequency electrical power is supplied to RF coil 13 so as to heat susceptor block 1'4- and silicon wafer 30 to a temperature of about 1200 C. Wafer 30 is maintained at this The invention will be described in greater detail in j Atemperature for about 15 minutes in an ambient of flowing pure dry hydrogen. The purpose of this step of heating the wafer in hydrogen is to clean the surface of Wafer 30 of any rlilrn of semiconductor oxides. A surface lm of silicon dioxide reacts with the bulk silicon under these conditions to form silicon monoxide, which is removed by evaporation and by reaction with the hydrogen ambient. Such removal of any semiconductor oxide lilm is advantageous for best results.

The electrical power supply is reduced, so that the temperature of wafer 30 drops to a temperature between about 1000 and about 1150"` C. The mixture of silane and hydrogen from cylinder 17 is then added to the hydrogen comingfrom source, 18,so that the gas mixture4 flowing through the reaction vessel 11 consists of about .01 to 1.5 volume percent silane, balance hydrogen. The silane decomposes in reaction vessel 11 in accordance with the equation. Y

SiH4 Sij-2H21` and deposits as an epitaxial layer 32 (FIGURE 3b) on wafer 30. Mixtures of silane and -inert gases `such as argon have 'been heatedto produce high-purity silicon, but mixtures of silane and hydrogen have not been used,

since as indicated above,"hydrogen is a decomposition produce of silane, and normally the presence of a high concentration of a. reaction product tends, to dr-ive the equilibrium state of a reversibler reactionl toward the left. The rate -of deposition yof the vepitaxial layer 32 depends on the concentration of silane on the reaction vese sel 11. as thiCkAasdeS-ired by varying the Vtime duringwhich the wafer `is heated in `the silanefhydrogen. mixture. In this example, the epitaxial layer 32 is about .5 mil thick,

Accordingly, the epitaxial layer 32 may be madeV and Yhas an electrical resistivity of about `30 ohm-cm: f

The valves of the silane-hydrogen cylinder 17 are closedy' the electrical power supply to RF coil 13 is interrupted,

and wafer 30 is permitted -to cool in an ambient of pureY Y lattice of silicon layer 32- is an extension of the crystalv lattice .of lwafer 30.

Referring now to FIGURE 3c, a P-conductivity type modiiier or acceptor is diifused into a limited portion 34 of epitaxial layer 32Vby standard techniques which are-known to the semiconductor art,1and need not be described here. Portion 34. of epitaxial layer 32 isthus converted to P-type and a rectifying barrier orpfn junction 35 is formed in the epitaxial ylayer 32 at the inter.-

Iface between .the'P-type region 34 and the N-type remainder of the layer. v

A portionr of the .epitaxial'layer 32 is masked with an acid resist (-not shown) such as apiezonV Wax or parafiin wax, and the wafer is then immersed in a suitable etohant, such as a mixture-of hydrofluo/ric acid andl nitric acids, :for a period of time suicient to remove thel unmasked portion of epitaxial layer32. The wafer is washed in distilled water, and the acid resist is removed by, means .of a suitable solvent, such as carbon tetrachloride, leaving the eched wafer (FIGURE 3d) with a mesa 37 which includes the remaining portion 32' ofthe epitaxial` layer, the remaining portion 34 of the P-type diffused region, and the remaining portion 35' of the rectifying barrier. An electrical lead Wire .38 is bonded, tothe top of mesa 37-'by'any convenient technique, such as soldering orfthermocompression bonding. The mesa diodeV device thus fabricated is then mounted andv encapsulated by methods known to the transistor art, such as those described in Transistor Technology, volumes I-III, D. Van Nostrand Co., Princeton, New Jersey, 1958.

Example II i In this example, a monocrystalline P-conductivity type l'wafer 40jy (FIGURE 4a) Vis prepared withV two opposing.

major faces 41and 43. Advantageously, at least one major face 41 vis lapped, polished, and etched so as to be smooth and flat. l

Wafer is positioned on susceptor'block 14 ofv apparatus 10 (FIGURE 2) with major face 41 uppermost. An epitaxial silicon layer 42 (FIGURE 4b) about 1 micron thick is deposited on major wafer face 41 in the manner described above inl-Example I, that is, by first evacuating` thereaction rvessel 11to a pressure below 0.1 micron Hg, `then purging the reaction vessel 11 with layer'42. Suitably, the doping agent mixtures in tanks 19 and 20 contain about 10 to 1000 parts per million by rvolume of a Vgaseous conductivity modifier in hydrogen. The amountof thismixture addedto the ambient owaround lthe Wafer is about .001to 1 volume percent. As a result, suicient 'arsenic atoms are incorporated in epitaxial layer 42 to `induce N-type conductivityrin-layer 42.'.v A rectifying barrier or p-n junction' Yis thereby formed at `or closetothe interface` 41 be.

tween P-type wafer 40 land N-type epitaxi'al; layer 42.

Referring now to FIGURE 4c, a layer .44 of phosphorus `doped silicon oxide is deposited on the'epi'taxial silicon layerV 42. i The silicon `oxide Ylayer. 44 maybe formed by Voxidizing the. surface of .the silicon layer, for

example, by heatingvthe wafer in "air or in steam. Al-v phorus .may be` incorporated inthe silicon -oxide layery duringthe deposition step,l `or maybe introduced thereinV Y by a diffusion step after the deposition step.` f

Predetermined portions of the phosphorus-diifused silicon'oxide layer 44 are nowy removed,leaving the wafer 40 with portions 44 of the doped oxide as illustrated in FIGURE 4d. The removalofselected portions of silicon oxide layer 44 may Vbe accomplished 'by `lapping tools or grinding wheels, or by masking and etchingtechniques. Wafert is then `heated in an ambient-of air vor oxygen forabout 1/2 hour atabout l100 C." During lthis step, phosphorus from the 'remaining' portions of the doped silicon oxide layer '44 diffusesthroughthe epitaxial layer 42. The portions 46 (FIGURE 4d) of epitaxial'layer 42..immediately beneathfthe dopedsilicon oxide portions 44' .are-converted to N+ conductivity. At the.;rinter -face A19-between phosphorus-diifused portions 'of epitaxia'l layer.42 andthe remainder of the llayer,.'an` N-N-ljunctionis for-med. The heating step alsoY causesthe formation of a fresh'silicon oxide layer 45 'on the sur- -face of epitaxial layerwtZ; f i Y' j Preselected portions of the silicon oxide layer .44 are then removed so as to exposeareas 55 and 57 of Ithe epitaxial layer 42l within' the phosphorus-(infused regions, leaving the wafer 40 as illustrated in FIGURE 4e. The removal may-be laccomplished vby lapping` or'fgrinding techniques,for bymeans of masking ande'tching processes. Metallic electrodes 51 and 53 (FIGURE 4f) are now deposited on the exposed areas 55 and 57 respectively of the'epitaxiallayer 42. The metallic electrodes may, for example, consistofl chromium and silver, and may be deposited by evaporation. Atk the same time,1V another metallic'electrode 52 is Vdeposited on thek silicon oxide layer 45 between contacts 51 and 53. To. complete the device, electricallead Wires 61,162 and @Bare lattached to `electrodes 51,52 and 53 respectively by any convenient method, such as by soldering'orby a thermocompression bond. v

The device thus fabricated isan insulated 'gate iield effect triode. In operation, electrode 51 mayserve as the source electrode, electrode 53'as the'drain electrode, and

electrode `52 Vasthe gate -or control'electrode. In the sym- Y rnetrical structure of this example,rthe source Vand drain .electrodes mayv beY exchanged. A. ThenN-type portion of theA epitaxialflayer.42V between electrodes 51--and y53 is the conducting channel. Whe/nv a positive bias is applied `to the gate electrode 52,.the`device operates in the current enhancement mode'. ,When negative fbias-is applied may 'be introduced into the epitaxial layer in a uniform fashion. This is advantageous when it is desire-d to avoid the concentration gradient produced by the conventional technique of introducing a conductivity modified into an epitaxial layer by the usual diffusion process. Moreover, a very abrupt junction may be formed by the processes described above between the substrate Wafer and the deposited epitaxial layer.

The above embodiments of the invention'h-ave been described by Way of example only, and not limitation. If desired, the conductivity type of the various Wafer regions described may be reversed, utilizing appropriate acceptors and donors. Other compatible crystalline semiconductors, such as silicon-germanium alloys and germanium, may be utilized as substrates instead of silicon. Silicon may also be deposited in this manner on metallic or insulating substrates. Other variations and modications may be made by those skilled in the art without departing from the spirit and scope of the instant invention as described in this specification and in the appended claims.

What is claimed is:

l. The method of forming an epitaxial silicon layer on a semiconductive Wafer, comprising heating said Wafer ata temperature of about 1000 to 1350 C. in an arnbient consisting essentially of hydrogen and 0.01 to 1.5 volume percent silane.

2. The method of forming an epitaxial silicon layer on a crystalline silicon Wafer, comprising cleaning the surface of said Wafer and Iheating said Wafer at a temperature of about l000 to 1350 C. in an ambient consisting essentially of hydrogen and 0.01 to 1.5 volume percent silane.

3. The method ol' forming an epitaXial silicon layer on a crystalline silicon Wafer, comprising etching one major face of said Wafer to leave a smooth surface on said Wafer and heating said Wafer at a temperature of about 1000 to 1350 C. in an ambient consisting essentially of hydrogen and 0.01 to 1.5 volume percent silane.

4. The method of forming an epitaxial silicon layer on a crystalline silicon Wafer, comprising the steps of etching at least one major face of said Wafer, to leave a smooth surface on said Wafer heating said Wafer to a temperature of about l200 C. for about l5 minutes in a owing hydrogen ambient, then maintaining the wafer temperature at about 1100" C. While adding about 0.01 to 1.5 volume percent silane to said lowing hydrogen ambient.

5. The method of forming an epitaxial silicon layer on a crystalline silicon Water, comprising the steps of etching at least one major face of said wafer, to leave a smooth surface on said Wafer heating said Wafer to a temperature of about 1200 C. for about 5 to 30 minutes in a ilowinghydrogen ambient, then maintaining the Wafer temperature at a temperature of about 1100 C. While adding to said owing hydrogen ambient about 0.01 to 1.5 volume percent silane and about .001 to l volume percent of a mixture of hydrogen and about 10 to 1000 parts per miliion of a gaseous conductivity modirier.

6. The method as in claim 1, in which said gaseous modifier is selected from the group consisting of BZHG, PHS and AsH3.

References Cited bythe Examiner UNITED STATES PATENTS 2,916,359 12/59 Ellis et al 23-223.5 2,993,762 7/61 Sterling et al 23-2235 3,078,150 2/63 Raymond Z3-223.5

FOREIGN PATENTS 745,698 2/ 5 6 Great Britain.

DAVlD L. RECK, Primary Examiner. 

1. THE METHOD OF FORMING AN EPITAXIAL SILICON LAYER ON A SEMICONDUCTIVE WAFER, COMPRISING HEATING SAID WAFER AT A TEMPERATURE OF ABOUT 1000* TO 1350*C. IN AN AMBIENT CONSISTING ESSENTIALLY OF HYDROGEN AND 0.01 TO 1.5 VOLUME PERCENT SILANE. 